FIG. 1 is a structural drawing of a highly luminous D2R-type discharge lamp for an automobile front lamp. This discharge lamp is constructed so that a metal iodide sealed in a quartz emissive section 5 emits light as a high tension is applied between the metal tungsten electrodes 1a and 1b. The emissive section 5 is covered with an external quartz tube 2. In comparison with a halogen lamp chiefly presently employed as the front lamp for automobiles, the halogen lamp of FIG. 1 advantageously provides a threefold luminosity at 70% consumed power. Besides, since unlike halogen lights, no filament is used in the lamp of FIG. 1, the service life is very long and not shorter than 1,500 hours.
In this discharge lamp, a light-screening film 4 is formed on the surface of an outer quartz tube 2 to control the projected light region. This light-screening film is about 20.mu.m thick, and the shape and size thereof are such to be in compliance with the International Standard. The present invention relates also to a method for producing a light-screening film to be placed on the discharge lamp.
Using ferric oxide or cupric oxide as a pigment and sodium silicate or aluminum phosphate as a binder, a conventional light-screening film for lamps has been formed by mixing the pigment and the binder to form a paint; this paint is then applied to the glass surface of a lamp and the coat is fired at a temperature of from about 100.degree. C. to about 250.degree. C.
When such a lamp is lit, the temperature of the glass surface of the lamp rises; accordingly, the temperature of the light-screening film formed thereon rises also. Especially with a highly luminous automobile discharge lamp, the temperature of the lamp's glass surface rises to about 700.degree. C. during the lighting and, accordingly, the light-screening film on the glass surface is exposed to a temperature of 700.degree. C. also.
As mentioned above, a light-screening film made in accordance with prior art techniques uses ferric oxide or the like as pigment. Although cupric oxide is black at room temperature, this oxide is known to turn into red powder as oxidation progresses at about 350.degree. C.
Thus, if a light-screening film is formed on a highly luminous discharge lamp for automobiles using prior art methods, the color of the light-screening film changes from black to red or white due as the temperature rises during lighting. When the color of the light-screening film turns from black to red or white, the absorbance of light changes, thereby leading to a decline in light-screening performance, which, of course, leads to various drawbacks. Discoloration of a light-screening film causes not only a decline in light-screening performance but the lamp also gives a bad appearance to the user; this, accordingly, has become a serious problem. Thus, there is a desire for a light-screening film material that undergoes no discoloration for a 1,500-hour lighting period.
Another problem results if a light-screening film is formed on a highly luminous automobile discharge lamp using prior art techniques. A heat cycle constituting ups and downs of temperature caused by the repetition of turning the lamp on and off causes cracking or peeling of the light-screening film, thereby resulting in loss of light-screening performance. To address this problem, a light-screening film material free of cracking or peeling due to lighting and extinction (turning the lamp on and off) over 1,500 hours is desirable.